ABSTRACT The majority of the 300,000 annual deaths worldwide from prostate cancer are strongly attributed to bone metastasis because the skeleton is the exclusive site of clinical disease in the majority of men with advanced illness. There is a fundamental gap in our knowledge of the mechanisms that underpin the efficient and life- threatening colonization of the bone microenvironment by prostate cancer cells. Our long-term goal is to develop novel therapeutic strategies based on a molecular understanding of the pathophysiology of bone metastases in order to significantly improve survival and quality of life outcomes in prostate cancer. Our laboratory studies implicate two different but functionally related integrins expressed by prostate cancer cells in their homing, survival and lethal spread within the bone microenvironment. The rationale of this study is that a bispecific antibody that simultaneously targets these 2 integrins would optimally neutralize their function via its cross-linking mechanism of action and deliver an efficacious therapeutic strategy. Accordingly, a first-in-class bispecific antibody targeting these integrins demonstrated superior antitumor activity compared to monospecific antibodies alone or in combination. A distinct molecular mechanism of action for the bispecific antibody was defined. Following treatment with either or both monospecific integrin antibodies, adaptive upregulation of the integrins was seen whereas by contrast, downregulation of integrins followed bispecific antibody treatment via induction of internalization and lysosomal degradation of integrins. Our hypothesis is that the bispecific integrin antibody will halt the life-threatening progression of prostate cancer in the bone microenvironment. We plan to evaluate this hypothesis with three specific aims. First, we plan to assess the efficacy of the bispecific antibody compared to monospecific antibodies in distinct animal models of bone metastases that replicate key dimensions of the clinical disease: seeding of the bone marrow, interaction with human bone-derived stromal cells, accelerated growth and secondary dissemination from bone, and finally, generation of an osteoblastic phenotype. Secondly, we will further define the mechanism of action of the bispecific integrin antibody by assessing its impact on epithelial-mesenchymal transition, anoikis and clonogenic survival. Finally, we will determine the organ-specificity of the expression of the two integrins by comparing the expression of these integrins in bone metastases from prostate cancer and other solid tumors to metastases found in lymph nodes and visceral organs. Our innovative therapeutic strategy to disable the molecular mechanisms of colonization of the bone microenvironment by prostate cancer is significant because it has the potential to significantly prolong survival and improve quality of life of patients with prostate cancer. Solid tumors that colonize bone such as breast cancer may leverage the same molecular pathways, expanding the potential significance and impact of the bispecific antibody strategy in biomarker-supported clinical trials to follow.